1. Field of the Invention
The present invention relates to optical fiber component, an optical coupler and a method of producing thereof.
2. Related Background Art
Because rare earth elements and transition metals such as erbium (Er) can amplify light, they can be used in optical fiber laser and optical fiber amplification. The light in this optical fiber propagates essentially through the core of the optical fiber, therefore, rare earth elements are normally added to the core. The rare earth elements pumped by exciting light have the ability to amplify light. However, such rare earth elements cause light loss in signaling light, and adjusting the quantity of rare earth elements doped into the core is extremely difficult. Even though the light in the optical fiber propagates essentially within the core, it partially leaks out through the cladding. This indicates the possibility to realize light amplification even when the cladding of the optical fiber is doped with rare earth elements. However in this case, as the cladding has a lower light power level than the core, it necessitates a high concentration of doping with rare earth elements.
A vapor phase epitaxy method and a solution impregnation method, etc. are known as methods to add rare earth elements as active element to the optical fiber. In the vapor phase epitaxy method, first, chloride of rare earth element is heated and the resultant vapor is allowed to flow into a reaction tube together with material gas of silica glass, for example, silicon tetrachloride. Then, glass is synthesized. Next, a rod is produced using the glass added with rare earth elements and after being made transparent a cladding material or a core material is formed to produce optical fiber. There is another method to form the optical fiber. In the method, first, a group of silica particles is heated and being made transparent. When the group of particles being made transparent, rare earth elements are added to the glass by exposing the group of silica particles to a vapor of rare earth elements.
In the solution impregnation method, silica particles are impregnated in an alcoholic solution or aqueous solution of rare earth elements, then by sintering the silica particle group after evaporating the solvent, rare earth elements are added to the glass, to obtain optical fiber.
However, in the methods of producing optical fibers described above, in the case of vapor phase epitaxy method, the doping concentration is limited to the range from several ppm to several tens ppm due to the low vapor pressure of rare earth elements. Accordingly, a satisfactory light amplification ability is not exhibited by the optical fiber which is produced by the method described above such as the cladding is doped with rare earth elements. In the solution impregnation method, higher concentration of several thousands ppm is possible. However because the impregnation quantity is dispersed due to apparent density distribution of the silica particle group, it is difficult to obtain an optical fiber in which the rare earth elements are added uniformly to the cladding in a high concentration.
Unlike the methods described above, a sol-gel method enables rare earth elements to be added in a high concentration and prevent concentration association. Accordingly when optical fiber is spun using a glass material obtained by the sol-gel method as a base material for the cladding, optical fiber components might be produced in which the rare earth elements is added uniformly to the cladding in a high concentration.
The light in the optical fiber propagates essentially within the core, but it partially leaks out through the cladding and thus propagates within the cladding, as hereinbefore described. Particularly if an optical fiber has a thinner portion in a part of the cladding portion, the light propagating within the cladding has a relatively large light power density at that thinner portion. Because of this, a relatively large light amplification ability might be exhibited by partially forming the optical fiber thinner even when active elements are added onto the cladding over the entire length of the optical fiber.
However, the light propagating within the cladding becomes relatively large at the portion where the optical fiber is partially formed thinner, thereby the light often leaks outside the cladding. At a portion in the cladding except for the thinner portion contributing to light amplification ability, propagated light of the cladding suffers loss due to rare earth elements. In particular, when the optical fiber components are doped with rare earth elements in a relatively long portion with higher concentration, the total light loss is not negligible, and causes difficulty in exhibiting satisfactory light amplification ability.